Scraper elevator



Dec. 15, 1964 Filed Oct. 11, 1962 J. H. CREIGHTON ETAL SCRAPER ELEVATOR 2 Sheets-Sheet 1 I00 92 BI FIG. 4

INVENTORS JOHN H.CREIGHTON 8 HAROLD C.WELSER ai/MM ATTORNEY 1964 J. H. CREIGHTON E TAL 3,1 0

J SCRAPER ELEVATOR Filed Oct. 11, 1962 2 Sheets-Sheet 2 INVENTORS JOHN H.CREIGHTON 8:

HAROLD C.WELSER ATTORNEY United States Patent ()fitice 3,161,280 Patented Dec. 15., 1964 3,161,286 SCER ELEVATGR John H. Creighton, Bettendorf, Iowa, and Harold C. Welser, Rock Island, llL, assignors to Deere & Company, Moline, Ill., a corporation of Delaware Filed Oct. 11, 1962, Ser. No. 229,797 12 Claims. (Cl. 198---109) This invention relates to an elevator used in an earth moving piece of equipment of the scraper type.

A scraper of this type is shown in US. Patent 3,028,694 which issued to Mr. John H. Creighton April 1962. The scraper there shown includes a box-type structure open at its forward end with the floor of the structure having a forward cutting edge adapted to cut the upper surface of the ground. Cooperating with the cutting edge is an inclined elevator of a flight type with a lower run thereof adapted to engage the material and to move it both rearwardly and upwardly. The elevator is composed of a pair of side frame members rigidly joined and extending the full length of the elevator. The side frames or beams carry sprockets at the lower forward and the upper rear ends which engage the chains of the flight-type conveyor. A major problem which arises in use of such an elevator occurs due to stones or rocks passing between the chains and the sprockets which, due to the rigidity of the elevator frame, will often times break either the chain or one of the chain sprockets.

With the above in mind, it is a primary object of the present invention to provide a new and novel type of flight-type elevator composed of an upper frame portion including a pair of side beams rigidly interjoined. Telescopingly extending from the forward ends of the side beams is a pair of arms carrying forward sprockets of the flight-type elevator. Rotary means in the form of sprockets at the upper end of the rear portion drive the chain around the lower sprockets. The arms at the lower end of the elevator are telescopingly received in the side beams of the upper frame portion and are independently mounted so as to move both relative to the opposite arm as well as the upper frame portion. Cushioning means is provided between the upper frame portion and the forwardly projecting arms so as to bias or retain the sprockets and arms at a maximum forward position but to yield so as to permit foreign objects to pass between the sprockets and chain.

It is a further object of the invention to provide a chain guide and take up device for each of the arms and side beams composed of a rigid elongated fore-and-aft extending element pivotally mounted on the respective arm closely adjacent the sprocket and extending rearwardly therefrom to a point adjacent the respective side beam. The upper edge of the rigid element engages the underside of the upper run of the chain. At the remote end of the rigid element is a guide sprocket for the chain. The rigid element is also connected to the side beam by a vertical link permitting the entire element and guide sprocket to move rearwardly as the forward arm retracts in the side beam. The link is adjustable both vertically and fore-and-aft relative to the side beam so as to provide take up due to wear and other elongation of the respective chain.

Other objects and advantages of the present invention will become apparent to those skilled in the art as the nature of the invention is better understood from the following description and as shown in the accompanying drawings.

FIG. 1 is a side perspective view of the right side of a tractor and a scraper.

FIG. 2 is an enlarged perspective view taken from the front and side and showing the forward portion of the scraper.

FIG. 3 is a side view of a portion of the lower end of the elevator.

FIG. 4 is an enlarged sectional view taken substantially along the line 4-4 of FIG. 3.

FIG. 5 is a sectional view taken substantially along the line 55 of FIG. 4.

FIG. 6 is a vertical sectional view taken substantially along the line 6-6 of FIG. 2.

The scraper herein to be described is of the type set forth in the aforementioned patent. Consequently the present description will be only general and will be detailed only to the matter of the present invention. Should a further detailed description of the basic scraper and tractor be desired such may be had by reference to US. Patent 3,028,694.

The scraper 20 is composed of an elongated dirt carrier or bowl formed by a pair of fore-and-aft extending upright side walls 21, 22. The side walls 21, 22 have superstructure or framework, such as at 23, forming with other such framework a main frame for the implement. The side walls 21, 22 are interconnected at their lower edges by a horizontally disposed floor composed of a front section 25 and a rear section, not shown. The front section 25 is provided with rollers, such as at 27, 28, on opposite edges that ride on longitudinally extending tracks, one being shown at 31, on lower edges of the right side wall 22. The front floor section 25 may slide under the rear floor section for purposes of discharging the material from the box. A hydraulically controlled linkage system, not shown, is retained at the rear of the scraper bowl for the purpose of moving the forward section 25 between its rearward and forward positions. The floor section 25 supp rts a centrally located cutting blade 34 which digs into and permits the dirt to move into the rearward portion of the scraper bowl 20. The bowl 2th is closed at its rear by a rear wall or ejector plate adjustable longitudinally as is conventional in this type of scraper. The bowl 2%? is carried at its rear by wheels 41 mounted on a rearwardly projecting frame 39.

A tractor 45 is provided to pull the implement and includes rear traction wheels 45 carried on axle structure 47. Supplied with the tractor is a power take-oft shaft, not shown, which operates a drive, shown partially at 48, 49 in FIG. 1. The drive system between the tractor and scraper is the subject matter of US. Patent 3,038,266 which issued to Clarence T. Rasmussen and John H. Creighton June 12, 1962. Details of the drive system may be had by reference to that patent.

The scraper bowl or body 2i) is connected to the tractor by means of a Y-shaped beam or yoke structure indicated in its entirety by the reference numeral Stl. The latter structure includes a pair of transversely spaced leg p0rtions 51, 52 interconnected at their forward ends by a transverse cross beam 53. Rear ends of the leg portions 51, 52 are pivotally mounted to the upper edges of the side walls 21, 22 so as to permit relative vertical movement between the beam structure and bowl 20. Projecting forwardly from the cross beam 53 is a longitudinally extending beam 54 having a hitch connection 55 at its forward end. The hitch connection 55 permits a wide range of articulation between the beam 54 and the tractor 45. Extending between the cross beam 53 and the forward lower ends of the side walls 21, 22 are hydraulic units 56, 57, controllable at the operators station on the tractor to adjust the depth of cut as well as the position of the bowl or scraper 20 relative to the ground. A cross beam 58 extends across the forward open end of the bowl and effects transverse rigidity at the open forward end.

The elevator, indicated in its entirety by the reference numeral 60, is positioned at the forward open end of the bowl 20 and generally operates in conjunction with the blade 34 to move earth both rearwardly and upwardly i tively at the lower end of the elevator. The chains are interconnected by flights 111.

Referring for themoment to the cushioning means 90 7 that biases the arms 80, 81 to their maximum forward upper portion of the frame also'includes a pair of rear- 7 wardly projecting arms, one of which is shown at 65, connecting'the upper portion to the upper edges of the side walls 21, 22. V I 1 Referring now to FIGS. 3, 4 and'5, the right side beam 62, which is identical to the left side beam 61', is composed of a rear outwardly opening U-shaped beam 70 and a forward outwardly openingU-shaped beam 71 offset inwardly and overlapping longitudinally the forward end of the beam 70. As shown in FIG. 5, the beam 71 is slightly larger than the beam 70 and conse-' quently slightly embraces the inner portion of the beam 70- and is welded, as at 72, to the beam 70. An upright plate 73 has its upper and lower edges welded at 75 to the inner surfaces of the upper and lower leg portions of the beam 71. The plate 73 also is fixed tothe inner surface of the bight portion of the beam-,70. The plate 73 is substantially the length of the beam, 71 and defines therewith a hollow rectangular and longitudinally extending opening.

Reciproc'atingly or telescopingly supported within the hollow rectangular openings of the side beams 61, 62 is a pair of identical forwardlyprojecting arms 80, 81. The. Y

' having leg portions extending to opposite sides and pivotally supporting at 85 an idler 'chain carrier 'or sprocket 8.6. A similar yoke structure 87 and idler carrier or sprocket 88 are provided on the leftarm 80.

Retained in the hollow rectangular cross sections of the side beam structures 61, 62 are cushioning means position, it should be recognized that at times the chains 1%, 110 will pick up large stones or foreign ob ects and passthe stones or objects between the sprockets on the I 'main drive shaft 108 or the sprockets 86, 88 and the chain. This will, of course, create rearward pressure 'on the. arms '80, 81 so as to compressthe cushioning means 90 until the stone or foreign objectpasses completely around the sprocket. At this time the cushioning means 90 will bias the arms 80, 81 and the. sprockets I 86, 88 to resume their maximum extended positions. It

should also be noted that since the arms 80,- 81 are-1m dividuallymounted within the side beams 61, 62, a stone may pass relative to either of thechains 109, 110 and their respective sprockets without effecting theopposite chain and sprocket supporting structure.- There is sufficient flexibility in the chains 109, 110 to permit one chainto operate normally while the otherlis passing a steneor foreign object between one of its "sprockets and the chain.

Guide and take up means is provided for each of the chains 109, 110 and includes-a pair of elongated r1g1d 1 elements 120, 121 normally underlying the upper runs of the respective chains 109,- 110. As may be seen from V viewing FIG. 6, the rigid element 121' is composed of a for the respective arms 80, 81, the cushioning means in e the right beam section 71 being indicatedin its entirety; by the reference numeral 90. Referring to FIGS. 3 and 4,

a vertical transverse plate 91' is welded at 92 to the forward arm 81 and extends across the opening formed 'by the beam or arm 81 and plate 73. Adjacent the rear portion of the beam 71 and welded tothe plate 73, as at 93, is a transverse plate 94 extending across the opening formed by the plate '73 and beam 81, to a point closely adjacent the bight portion of the U-shaped for.- ward arm or beam 81. The cushioning device is composed of end plates 95, 96, normally abutting the respective plates 91, 94 and a series of rubber wafers 97 longitudinally separated by metal plates 98. A rod 100 extends through the center of the rubber wafers 97 and plates 95, 96, and 98 and generally retains the plates:

and disks in'a unified condition, nuts 101, 102 being provided on opposite ends thereof'and bearing against the end plates 95, 95. Central openings are provided in the plates 91, 94 to permit passage of; the rod 100 and the respective nuts 101, 102. A forward stop bracket 105; is bolted at 106 to the plate 73*and will normally abut against the front surface of'the plate 91 so as to provide a maximum forward position that the arm 81 may extend. Also, the stop reinforces the plate 91 when pressure is applied to the rear side beam 62 in a forward direction, A pair of g'ussetplates 107 extends from the plate 73 to the rear surface of the transverse plate 94 to reinforce the latter..

Providedadjacent the upper end of the rear side beams The flight conveyor 110 extending over the rotary guide means at the upper endof the elevator andover the sprockets 88,!86-16SP8G- engaging the upper runs'of the chains 109,110.

elements 120, 121 therefore generally underlie the lower pair of upright sides 123, 124 rigidly joined by a spacer 125. Upper edges of the sides 123, 124. contact the underside of the individual links forming the chain 110.

Rigid elongated elements 120, 121 are pivotally mounted at 130, 131 to forward portions of the arms 80,81. Referring to FIG. 3 and relative to the element 121, an upright bracket 133 is fixed to the forward edge of the arm 81 and between the-leg portions of the yokeSZl The forward edge of the bracket 133 is arcuate shaped as at 134 and lies adjacent to the rear periphery of the sprocket '86. The purpose of locating the bracket 133 at the particular location is generally to prevent rocks from entering into the area between the chain and sprocket 86'. The rigid-elements 120, 121 extend rear- Wardly from their forward pivots 130, 131 to remote ends well beyond the midportion of the longitudinal length of the elevators. The remote ends are bifurcated, as at 134, 135 and extend downwardly. The ends 1 34} 135 support idler sprockets or chain guides 136, 137 The portions of'the-upper runs of the chains 109, 110. This will in most instances prevent stones from entering into the area between the lower sprockets 88, 86"andthestructure 140 is provided with a row or series of longitw. dmally and vertically spaced holes 141. A row of holes 142 is provided in the rigid element121- and generally above the holes 141. An upright link 144 is pivotally connected at 145, to'one of the holes 141 and to" one of the holes 142; By moving the link 144 forwardly to forwardlydisposed holes 141, 142, it becomes apparent that the rear end 1350f the rigid element 12'1fWill rise.

Therefore, shoulditbe necessary to provide take .up in the chain due to. wear'or other lengthening'eflects of the chain110,'such may be done by merely repositioning the link to'difierent holes 141, 14 2. A lso',it should be recognized that by mounting the element 121 on the forward arm 81, should'for any reasonthe cushioning means- 9:8v be compressed, the entire guide element 121 and its sprocket 137 will merely move rearwardlyr The link; 144-" will permit such movement. Consequently at all times is the guide characteristic of the sprocket 137 and element 121 retained.

While only one form of the invention has been shown, it should be recognized that other forms and variations will occur to those skilled in the art. Therefore while the present disclosure was made in detailed manner, it was so done to fully explain the principles of the invention and it was not the intention to limit or narrow the invention beyond the broad concepts set forth in the appended claims.

What is claimed is:

1. An elevator for use with an earth scraper having a forwardly opening material carrying bowl with upwardly extending opposed sides and a forward horizontally disposed cutting blade, the elevator comprising: a main frame inclined from a lower forward end to an upper rear end and composed of an upper laterally disposed portion having a pair of fore-and-aft extending side beams rigidly joined together by transverse framework, each of the side beams being disposed alongside a respective side; a pair of lower arms projecting forwardly from the respective side beams and joined with the side beams to permit inependent fore-and-aft reciprocation of the arms; rotary drive means at the upper end of the laterally disposed portion; a sprocket on each of the arms; a flight type conveyor including a pair of continuou chains carried by the sprockets and rotary drive means and interjoined by transverse flights; a cushioning structure between the side beams and arms for biasing the arms forwardly toward a maximum extended position and yieldable to permit the respective arms and sprockets to retract relative to the side beams; an elongated rigid element underlying and normally engaging the underside of the upper run of each of the chains and pivotally connected at its forward end to the respective arm at a point closely adjacent the sprocket, the element extending rearwarly from its forward end to a remote end adjacent the respective side beam; a guide sprocket on each of the remote ends engaging the chain; and a link extending from each of the side beams to rear portions of the rigid elements permitting the rigid elements to move rearwardly as the arms are retracted.

2. An elevator for use with an earth scraper having a forwardly opening material carrying bowl with upwardly extending opposed sides and a forward horizontally dis posed cutting blade, the elevator comprising: a main frame inclined from a lower forward end to an upper rear end and composed of an upper laterally disposed portion having a pair of fore-and-aft extending side beams rigidly joined together by transverse framework; a pair of lower arms projecting forwardly from the respective side beams and joined with the side beams to permit independent fore-ande-ft reciprocation of the arms; rotary drive means at the upper end of the laterally disposed portion; a rotary chain carrier on each of the arms; a flight type conveyor including a pair of continuous chains carried by the carriers and rotary drive means and interjoined by transverse flights; and a cushioning structure between the side beams and arms for biasing the arms forwardly toward a maximum extended position and yieldable to permit the respective arms to retract relative to the side beams.

3. An elevator for use with an earth scraper having a material carrying bowl with upwardly extending opposed sides and a forward horizontally disposed cutting blade, the elevator comprising: a main frame inclined from a lower forward end adjacent the blade to an upper rear end and composed of an upper laterally disposed portion having a pair of fore-and-aft extending side beams rigidly joined together by transverse framework; a pair of lower arms projecting forwardly from the respective side beams and joined with the side beams to permit independent fore-and-aft movement of the arms relative to their respective side beams; rotary drive means at the upper end of the laterally disposed portion; a chain carrier on each of the arms; a flight type conveyor including a pair of continuous chains carried by the carriers and rotary drive means and interjoined by transverse flights; a cushioning structure between the side beams and arms for biasing the arms forwardly toward a maximum forward position and yieldable to permit the respective arms to retract rearwardly toward the respective side beams; an elongated rigid element underlying the underside of the upper run of each of the chains and pivotally connected at its forward end to the respective arm at a point closely adjacent the respective carrier, the element extending rearwardly from its forward end to a remote end adjacent the respective side beam; a chain guide on each of the remote ends engaging the chain; and a connection between the side beams and rear portions of the rigid elements permitting the rigid elements to move rearwardly as the arms are retracted.

4. An elevator for use with an earth scraper having a material carrying bowl with a forward horizontally disposed cutting blade, the elevator comprising: a main frame inclined from a lower forward end adjacent the blade to an upper rear end and composed of an upper laterally disposed portion having a pair of fore-and-aft extending side beams rigidly joined together by transverse framework; a pair of lower arms projecting forwardly from the respective side beams and joined with the side beams to permit independent fore-and-aft movement of the arms relative to their respective side beams; rotary drive means at the upper end of the laterally disposed portion; a rotary chain carrier on each of the arms; a flight type conveyor including a pair of continuous chains carried by vthe chain carriers and rotary drive means and interjoined by transverse flights; a cushioning structure between the side beams and arms for biasing the arms forwardly toward a maximum forward position and yieldable to permit the respective arms to retract rearwardly toward the respective side beams; an elongated rigid element underlying the underside of the upper run of each of the chains and pivotally connected at its forward end to the respective arm at a point closely adjacent the respective sprocket, the element extending rearwardly from its forward end to a remote end adjacent the respective side beam; a chain guide on each of the remote ends engaging the chain; and vertical supports between the side beams and rear portions of the rigid elements permitting the rigid elements to move rearwardly as the arms are retracted, the vertical supports being adjustable to effect take up in the upper runs of the chains.

5. An elevator for use with an earth scraper having a material carrying bowl with a forward horizontally disposed cutting blade, the elevator comprising: a main frame inclined from a lower forward end adjacent the blade to an upper rear end and transversely spaced side structures; rotary drive means at the upper end of the frame; transversely spaced chain carriers on the lower end of the elevator; a flight type conveyor including a pair of continuous chains carried by the carriers and rotary drive means; an elongated rigid element underlying in closely adjacent relation to the underside of the upper run of each of the chains and pivotally connected at its forward end to a side structure at a point adjacent the respective chain carriers, the element extending rearwardly from its forward end to a remote end adjacent the respective side structure; a chain guide on each of the remote ends engaging the chain; and vertical supports between the side structures and rear portions of the rigid elements, the vertical supports being adjustable to effect take up in the upper runs of the chains.

6. An elevator for use with an earth scraper having a material carrying bowl with a forward horizontally disposed cutting blade, the elevator comprising: a main frame inclined from a lower forward end adjacent the blade to an upper rear end and transversely spaced side structures; rotary drive means at the upper end of the frame; transversely spaced chain carriers on the lower end of the elevator; a flight type conveyor including a pair of continuous chains carried by the carriers and rotary drive means; an elongated chain guide element underlying in closely adjacent relation to the underside of the upper ru'n of each of the chains andpivotally con-- nected at its forward end to a side structure at a point adjacent the respective carrier, the element extending. rearwardly from its forward end toa remote end adjacent the respective side structure; and vertical supports between the side structures and rear portions of the guide elements, the vertical supports being adjustable to effect take up in the upper runs of the chains.

7. The invention defined in claim 16 further characte'rized by the vertical supports being vertical links having pivotal connecting means at their upper and lower ends to the side structures and guide elements respectively with atleast one of the latter means being adjustable to effect 7 varying vertical spacings between the guide elements and the respective side structures.

8. An elevator for use with an earth scraper having a ,7

material carrying bowl with a forward horizontally disposed cutting blade, the elevator comprising: a main frame inclined from a lower forward end spaced side structures; rotary drive means at the upper end of theframe; transversely spaced rotary chain carriers on the lower end of the frame; a flight type conveyor 1 including a pair of continuous chains carried by the carriers and rotary drive means; an elongated chain guide element underlying in closely adjacent relation to the underside of the upper run of each of the chains and pivotally connected at one end to a side structure at a point adjacent the respective carrier, the element extend ing from the one end -to aremote end above the respective side structure; and vertical supports between the side walls rigidly joined together by transverse framework and a pair of arms'projecting fore and aft from the re-' .spectivesidesand joined with the sides to permit independent fore-and-aft movement of the arms relative to the respective sides; rotary drive means on the laterally disposed portion; a chain carrier on eachfof the arms;

a flight type' conveyor includinga pair of continuous chains carried by the carriers and rotary drive means and interjoinedby transverse flights; a cushioning'structure' between the sides and arms for biasing the arms toward a maximum extended position and yieldable to per- 'mit the respective arms to retract toward the respective sides; a rigid element having a part, thereof underlying the upper run of each'of' the chains and pivo tally con nected to the frame to move vertically; a chain guide for each of the chains supported o'n the respective part and engaging the underside of the respective upper runs;land adjustable means between the frame and element for adjusting the vertical position of the respective chain guides.

llaAn elevator for use with earth scraper having a material carrying bowl with upv'vardly extending opposed side walls and a forward horizontally disposed F cutting bladeptlie elevator comprising: a main frame inadjacent the 7 blade to an upper rear end and including transversely.

structures vand rear portions of the guide elements, the;

vertical supports being adjustable to effect take up in the upper runs of the chains. w

9. An elevator for use with an earth scraper havlng a forwardly opening material carrying bowl with a for- Ward horizontally disposed cutting blade, the. elevator comprising; a main frame inclined from a lower forward end to an upper rear end and composed of an upper laterally disposed portion; a pair of transversely ispaced lower arms projecting forwardly from the lateral portion and joined with the portion to permit independent foreclined from a lowersforw'ard end adjacent the blade to an upper rear endand composed of a laterally disposed main portion having opposite sides adjacent the respective side walls rigidly joined together by transverse framework and a pair of arms projecting fore and aft from the respective sides and joined with the sides to permit independent fore-and-aft mt'n'zerne'rit of the arms relative to the respective sides; rotary drive means on the laterally disposed portioii; a chain carrier on each of the arms; a flight'type conveyor including a pair of continuous chains carried by the carrierfs and rotary drive .means vand lnterjolned bytransverse flights; a cushioning strucinclined from a lower forward end adjacent the blade to and-aft movement of the arms; rotary drive means at the upper end of the laterally disposed portion; .a chain car-x rier on each of the arms; a flight type conveyor including a pair of continuous jchain's carried by the" carriers and rotaryltdrive means and. interjoined' by transverse flights; and acushioning structure between the later ally disposed portion and arms for biasing the arms toward a maximum extended position and yieldable to permit the an upper rear end and composed of a laterally disposed main portion having opposite sides rigidly joined together by transverse framework and a pair of arms projecting fore and aft from the respective sides and joined with the sides to permit-independent fom-and-a'ft movement I of the arms relative to their, respective sides';rotarydriv e means on the laterally disposed portion; a chain carrier "pair offcont i'n'uous chains carried by the carriers and respective arms to retract relative to the laterally disposed portion.

10. An elevator cutting blade, the elevator comprising: a mainframe inclined from a lower forward end adjacent the blade to an upper rear end and composed of a laterally dis posedmain portion having opposite sides adjacent the respective side for use with an earth scraper having a material carrying bowl with upwardly extending opvposed side walls and a forward horizontally disposed on each of the arms';'a'flight type'conveyor including a rotary drive means'an d interjoined by transverse flights; and a cushioning structure between the sides and arms for biasing the arms toward a maximum extended posi tion and yieldable to permit the respective arms to re tract toward the respective sides.

No references cited. 

1. AN ELEVATOR FOR USE WITH AN EARTH SCRAPER HAVING A FORWARDLY OPENING MATERIAL CARRYING BOWL WITH UPWARDLY EXTENDING OPPOSED SIDES AND A FORWARD HORIZONTALLY DISPOSED CUTTING BLADE, THE ELEVATOR COMPRISING: A MAIN FRAME INCLINED FROM A LOWER FORWARD END TO AN UPPER REAR END AND COMPOSED OF AN UPPER LATERALLY DISPOSED PORTION HAVING A PAIR OF FORE-AND-AFT EXTENDING SIDE BEAMS RIGIDLY JOINED TOGETHER BY TRANSVERSE FRAMEWORK, EACH OF THE SIDE BEAMS BEING DISPOSED ALONGSIDE A RESPECTIVE SIDE; A PAIR OF LOWER ARMS PROJECTING FORWARDLY FROM THE RESPECTIVE SIDE BEAMS AND JOINED WITH THE SIDE BEAMS TO PERMIT INDEPENDENT FORE-AND-AFT RECIPROCATION OF THE ARMS; ROTARY DRIVE MEANS AT THE UPPER END OF THE LATERALLY DISPOSED PORTION; A SPROCKET ON EACH OF THE ARMS; A FLIGHT TYPE CONVEYOR INCLUDING A PAIR OF CONTINUOUS CHAINS CARRIED BY THE SPROCKETS AND ROTARY DRIVE MEANS AND INTERJOINED BY TRANSVERSE FLIGHTS; A CUSHIONING STRUCTURE BETWEEN THE SIDE BEAMS AND ARMS FOR BIASING THE ARMS FORWARDLY TOWARD A MAXIMUM EXTENDED POSITION AND YIELDABLE TO PERMIT THE RESPECTIVE ARMS AND SPROCKETS TO RETRACT RELATIVE TO THE SIDE BEAMS; AN ELONGATED RIGID ELEMENT UNDERLYING AND NORMALLY ENGAGING THE UNDERSIDE OF THE UPPER RUN OF EACH OF THE CHAINS AND PIVOTALLY CONNECTED AT ITS FORWARD END TO THE RESPECTIVE ARM AT A POINT CLOSELY ADJACENT THE SPROCKET, THE ELEMENT EXTENDING REARWARLY FROM ITS FORWARD END TO A REMOTE END ADJACENT THE RESPECTIVE SIDE BEAM; A GUIDE SPROCKET ON EACH OF THE REMOTE ENDS ENGAGING THE CHAIN; AND A LINK EXTENDING FROM EACH OF THE SIDES BEAMS TO REAR PORTIONS OF THE RIGID ELEMENTS PERMITTING THE RIGID ELEMENTS TO MOVE REARWARDLY AS THE ARMS ARE RETRACTED. 